In the drilling and/or producing of an earth borehole steel casing may be routinely used in one or more sections of the borehole, and cement is employed on the outside of the casing to hold the casing in place and to provide a degree of structural integrity and a seal between the formation and the casing.
There are various circumstances in which it is necessary or desirable to make one or more perforations through the casing and cement in order to perform tests behind the casing, and through the surrounding cement, if present. For example, a commercially used technique employs a tool which can be lowered on a wireline to a cased section of a borehole, the tool including a shaped explosive charge for perforating the casing, and testing and sampling devices for measuring hydraulic parameters of the environment behind the casing and/or for taking samples of fluids from said environment.
After testing through perforations in casing, it is sometimes decided to perforate the well for production or to abandon and plug the zone. The term "plugging" traditionally means plugging an entire cross section of the well. Perforations can be plugged with cement through drill pipes. Elastomeric plugging is also used to plug an entire well by isolating the zone below the plug during or after the production. Elastomeric plugs are also used as an anchor for setting cement. Well treatment and plugging can also be done with coiled tubing.
A drawback of using a tool that perforates casing for testing is that the perforation which remains in the casing can cause problems in cases where production or zone plugging does not quickly follow. In some fortunate instances the perforation may become clogged with debris from the borehole and rendered essentially harmless if the debris permanently plugs the perforation. However, if the perforation, or part of it, remains open, a substantial volume of formation fluids may be lost into the formations and/or may degrade the formations. In some situations, fluids from the formations may enter the borehole with deleterious effect. Gas intrusion into the borehole can be particularly problematic.
It is among the objects of the present invention to address the problems of perforating and testing in cased sections of an earth borehole, and to devise an apparatus and method which solves the problem in a practical way.